Does Hot Tub Water Evaporate?




You may sometimes be ready to take a nice dip in your hot tub, only to notice that the water level in the tub is too low, and you need to top it off first. 

Alternatively, if you own a spa business and your hot tubs are in frequent use, you may have noticed that the water level drops significantly by the end of the day. All this might prompt the question, does hot tub water evaporate?

The short answer is yes. All water evaporates in any open-top vessel at any temperature. The question worth asking, however, is how much or how quickly does water evaporate?

Does Hot Tub Water Evaporate

Why does the water in hot tubs evaporate?

To understand why the water in your hot tub may evaporate over time, we need to understand some physics. This won’t be too complicated, I promise!

Even if water is not at boiling point, over time, due to the random movement of water molecules, microscopic droplets of water end up evaporating and leaving the container, or in this case, the hot tub. Due to this fact of nature, there will always be evaporation to some extent as long as the water is present in an open-top container. 

If the container is closed, such as a hot tub with a cover or lid, less evaporated water will be lost as it will simply condense on the lid and drip back into the hot tub. This is why water bottles, when left closed for a long time, end up with small droplets near the top. That is the water that would have otherwise evaporated but was condensed because the lid was closed. 

If the hot tub is outside and the cover is left off for a while, particularly during windy weather, the rate of evaporation will be higher.

How fast does hot tub water evaporate?

How much water evaporates from a hot tub

Now that we have established that water in hot tubs tends to evaporate, another question worth exploring is how much water this is. If only a few drops evaporate, then there is not much of a reason for concern. 

However, this is not the case. There are a few factors that dictate the speed at which water naturally evaporates. Depending on these factors, the hot tub could lose anything between 1 inch and 3 inches in a month.

Hot tub water surface area

Firstly, the surface area of the top of the water. In the case of a hot tub, which is open top, there is a large area where the water meets the air. Thus, this increases the rate of evaporation. 

Hot tub water temperature

Secondly, the temperature of the water. We all know that water has a boiling point, and it instantly evaporates when it reaches that temperature. However, water evaporates at lower temperatures too, and this speed increases the higher the temperature. 

Given that hot tubs are hot, it is safe to assume that the high temperatures will accelerate the rate at which the water evaporates from the hot tub. 

The humidity of the air

Finally, the local humidity level. Essentially, the higher the humidity level is, the slower the water evaporates. If the air is very humid it means the amount of water vapor in the air is also high (obviously) so it can absorb less water vapor. Once the air reaches saturation level it can absorb no more.

If your hot tub is outdoors in an area with a breeze, the humidity level is likely to be lower and evaporation faster. 

However, if the hot tub is indoors in a room that is not air-conditioned and lacks ventilation, you can expect evaporation to be slower as humidity would be higher. 

One thing to note is that the longer your hot tub is in operation, the more water it will lose. This is because the temperature of the water and the jets all impact the rate of evaporation, although the hot tub will lose water to evaporation even when it is idle.

How do I stop my hot tub water evaporating?

Hot tub cover

How do I stop my hot tub water evaporating

Covering the hot tub with your hot tub cover whenever it is not in use will help to reduce the amount of water lost through evaporation considerably.

As stated above, much of the evaporated water will condense on the underside of the cover and drip back into the hot tub.

If you do not have a hot tub cover then I would suggest that you buy one. Not only will this save water loss but, perhaps even more importantly, it will help reduce your electricity consumption by maintaining the hot tub’s temperature.

If you do not have a cover you can obtain expert advice and buy a qood quality cover for your hot tub with free delivery to many US states from Florida Spa Covers.

Florida Spa Covers can supply custom hot tub covers to your specification or by giving them the make and model of your hot tub they can supply one made specifically for it. Check out Florida Spa Covers website.

You can also buy replacement hot tub covers on Amazon – various are available to choose from.

Hot tub thermal blanket

Spa Depot Thermo-Float 16-mil 7ft x 7ft Hot Tub Bubble Cover Floating Spa Blanket - trimmable Heavy-Duty Insulating Solar Heating

A hot tub thermal blanket is very similar to a solar pool cover, only smaller to fit the hot tub.

It is made of plastic with air bubbles, similar to bubble wrap but more durable. It floats on top of the hot tub water.

It has two functions:

  • To trap heat in the water to reduce the heating costs
  • To prevent, or at least vastly reduce, evaporation

Although these can be used on their own, they will work more effectively if they are used with a hot tub cover.

They can be bought quite cheaply on Amazon. You can find the one shown above here.

How else can a hot tub lose water?

One thing to note is that evaporation cannot usually account for all the water a hot tub loses. In general, you are likely to lose water from your hot tub in a few other ways. 

For example, you may lose water to the splashing and the misting effect caused by the jets in the hot tub. In this case, the water is not being evaporated. Rather, it is lost as spray and goes into the general vicinity. 

Another reason you can lose water is because of people entering and exiting the hot tub. The splashing caused by an entrance can make some water pour out of the tub. Similarly, a wet person leaving the tub would take some water with them which they will deposit on a towel when they dry themselves.

Children have a habit of splashing around which obviously causes water loss too.

Together, these three factors will contribute significantly to the overall amount of water lost by your hot tub. 

If your hot tub is losing water overnight, and a significant amount, then the likelihood is that you have a leak which could be in the plumbing or you may have a crack in the hot tub shell.

Can water loss in a hot tub cause problems?

You might now be wondering whether the water loss can cause issues with your hot tub properly functioning. The answer is yes, it definitely can if it is not dealt with. 

When the hot tub’s water level is too low when the hot tub pump and heater are running, it might cause major problems. This is because a low water level could cause air to be drawn into the filters and jets instead of water. 

Both overheating and damage to these delicate components may occur from this. Long-term, this could mean that the hot tub’s parts may need to be changed. 

Even if the water level has not dropped to such an extent that damage may occur, if the hot tub water level falls below the jets, when the pump is on full water will shoot out of the jets and probably outside the tub.

The amount of water lost must be replaced so that the hot tub does not run on too little water for too long. 

How often should you top-up a hot tub?

Generally speaking, you want to look at your hot tub’s water level once per week. If it is below the required amount, you can go ahead and top it up. 

If your hot tub is outside and doesn’t have a cover then you may need to top it up more often as that is when evaporation will occur most quickly.

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